Vibration sensor using the capacitance between a substrate and a flexible diaphragm

ABSTRACT

A vibration sensor includes a substrate (7), a plate-like diaphragm (3) forming a capacitor, and holding assembly (2), (4), (6) for holding the diaphragm so as to be movable in parallel to the substrate. The diaphragm includes a plurality of flexible displacement portions (3c), (3d) for enabling the supporting portion of the diaphragm to be movable in parallel to the substrate. When vibrations are applied to the vibration sensor, the diaphragm portion of the diaphragm is vibrated and electrostatic capacity formed between the substrate and the diaphragm is changed based on the vibrations. Vibrations are detected on the basis of the change of electrostatic capacity. Since the vibration sensor includes the flexible displacement portions, the diaphragm becomes difficult to be vibrated due to a sound pressure and a vibration detected output can be prevented from being disturbed by the sound pressure.

This application is a continuation of application Ser. No. 08/592,397filed Feb. 1, 1996, now abandoned, which is a 371 of PCT Appln. No.PCT/JP95/01184, filed Jun. 14, 1995.

TECHNICAL FIELD

The present invention relates to a vibration sensor and a navigationapparatus. More particularly, this invention relates to a vibrationsensor for detecting vibrations based on a change of electrostaticcapacity and a navigation apparatus using such a vibration sensor.

BACKGROUND ART

Heretofore, there have been developed vibration sensors using anelectret film. As shown in FIG. 1, a vibration sensor includes a closedcase 30 containing a back plate 31 with an electret film formed thereon.A diaphragm 32 opposed to the electret film of the back plate 31 isdisposed with a very small clearance between it and the back plate. Theback plate 31 is fixed within the case 30. Electric charges areaccumulated between the electret film of the back plate 31 and thediaphragm 32. In this case, the diaphragm 32 is formed of a thin metalplate and is supported so as to be easily vibrated. In principle, thevibration sensor shown in FIG. 1 senses how much electric field changeswhen the diaphragm 32 is vibrated. The result is a voltage signal thatindicates an amount of vibrations applied to the sensor.

Specifically, if the diaphragm 32 is fixed, then the vibration sensor isvibrated together with a monitored object to which the vibration sensoris attached. When the vibration sensor is vibrated together with themonitored object, the back plate 31 is vibrated relative to thediaphragm 32. As a result, the electrostatic capacity between the backplate 31 and the diaphragm 32 is changed based on vibrations of the backplate 31. The detected change of the electrostatic capacity is output bythe vibration sensor. Although it is considered that the diaphragm 32 isfixed, if the vibration sensor is fixed together with the detectedobject, then it is natural that the diaphragm 32 should be vibratedbased on vibrations applied to the vibration sensor. In any event, thevibration sensor detects vibrations applied to the monitored object fromthe outside or vibrations generated in the monitored object as thechange of electrostatic capacity accumulated between the diaphragm 32and the back plate.

The back plate 31 and the diaphragm 32 are accommodated within theclosed case 30 in order to prevent the diaphragm 32 from being vibratedby a sound pressure based on sounds applied to the vibration sensor fromthe outside so that the vibration sensor is able to detect onlyvibrations accurately. Specifically, if the diaphragm 32 is vibrated inresponse to sounds applied to the sensor, then the vibration sensorregisters vibrations together with sound and the output from thevibration sensor contains an error component. In order to prevent thisdefect, the back plate and the diaphragm should be almost completelyenclosed by the case 30. FIG. 2 is a graph showing a sound pressurefrequency characteristic of the closed type vibration sensor. Since thecase 30 is of the closed type, the vibration sensor responds to only asound or sound pressure having a relatively low frequency and is unableto respond to a sound or sound pressure having a frequency higher than acertain frequency. When the vibration sensor is applied to a navigationapparatus in a vehicle, the vibration sensor tends to detect soundsgenerated in the vehicle, e.g., an automobile. There are then theproblems that a position on a correct map cannot be displayed and that acorrect position cannot be displayed on a map due to the error componentfrom the vibration sensor.

Study of characteristics shown in FIG. 2 reveals that the sensorinevitably responds to the sound pressure of low frequency soundsapplied to the vibration sensor and generates an erroneous output. Forthis reason, it is proposed to form a through-hole on the diaphragm.Specifically, when a response of the diaphragm relative to a soundpressure is in the state shown by the solid line in FIG. 3, if athrough-hole is defined at the central portion of the diaphragm, then aminimum frequency response to sound pressure can be shifted to a higherfrequency as shown by the broken line in FIG. 3. The minimum frequencyresponse to sound pressure can be further shifted to a higher frequencyas an area of the through-hole increases.

If the minimum frequency response to the sound pressure generated basedon sounds applied to the diaphragm from the outside is raised by using athrough-hole as described above, as shown in FIG. 2, then the diaphragmresponds to only a sound pressure having a frequency higher than a bandresponsive to the sound pressure of the low frequency shown by the soundpressure frequency characteristic of the closed-type vibration sensor.As a result, the diaphragm responds only to vibrations applied to thesensor and the sensor can indicate those vibrations by the outputsignal.

If however the through-hole is defined at the central portion of thediaphragm to lower sensitivity to sound pressure with a low bandfrequency, there is then the disadvantage that the detection sensitivityof the vibration sensor is lowered. Specifically, the level at which thediaphragm without a through-hole detects vibrations shown by the solidline in FIG. 4. Then when the through-hole is provided in order to lowersensitivity to sound pressure of the low frequency, the output level ofthe detection signal is lowered as shown by the broken line in FIG. 4.The output level is lowered much more as the area of the through-hole isincreased. That is, the through-hole decreases an area in which thediaphragm and the back plate oppose each other, lessening a usability ofelectric charges of the electret film. As a result, the output level ofthe detection signal is lowered.

Therefore, this kind of conventional vibration sensor cannot raise avibration detection sensitivity to a desirable extent.

DISCLOSURE OF THE INVENTION

In view of the aforesaid problem, it is an object of the presentinvention to provide a vibration sensor with a satisfactorycharacteristic capable of detecting only vibrations with a highsensitivity and a navigation apparatus using such vibration sensor.

According to a first embodiment, a vibration sensor comprises asubstrate, a plate-like diaphragm including a diaphragm portion, asupporting portion and a plurality of flexible displacement portions forcoupling the diaphragm portion and the supporting portion such that thediaphragm portion and the supporting portion become able to move inparallel to the substrate, the plate-like diaphragm being opposed to thesubstrate with a predetermined interval and being disposed in parallelto the substrate to form a capacitor, and holding means for holding thesubstrate and holding the supporting portion of the diaphragm in such amanner that the diaphragm portion comes close to or away from thesubstrate in a parallel direction, wherein a vibration is converted intoa change of electrostatic capacity formed between the diaphragm and thesubstrate and output as a detected output. With this arrangement, whenthe diaphragm is vibrated by vibrations applied to the vibration sensor,a detected electrostatic capacity is changed and therefore vibrationscan be detected satisfactorily.

According to a second embodiment, in the vibration sensor according tothe first invention, the plurality of flexible displacement portions areradiated from a center of the diaphragm portion. With this arrangement,the diaphragm can be vibrated satisfactorily and therefore vibrationscan be detected satisfactorily.

According to a third embodiment, in the vibration sensor according tothe second embodiment, the diaphragm includes between it and theplurality of flexible displacement portions means for canceling a soundpressure applied to the vibration sensor. With this arrangement, evenwhen sounds are applied to the vibration sensor from the outside, thediaphragm can be prevented from being vibrated and therefore onlyvibrations applied to the vibration sensor can be detected.

According to a fourth embodiment, in the vibration sensor according tothe third embodiment, the canceling means is composed of a plurality ofopening portions disposed along a circumferential direction of thediaphragm portion disposed between the plurality of flexibledisplacement portions. With this arrangement, it is possible tosatisfactorily cancel a sound pressure applied from the outside.

According to a fifth embodiment, in the vibration sensor according tothe first embodiment, the holding means is a housing for accommodatingtherein the substrate and the diaphragm in substantially closed stateand holding the substrate and the supporting portion of the diaphragm.With this arrangement, the inside of the sensor can be prevented frombeing smudged by dusts or the like. Moreover, the diaphragm can beprevented from being vibrated by a sound pressure based on soundsapplied to the sensor. Thus, vibrations can be detected satisfactorily.

According to a sixth embodiment, a vibration sensor comprises asubstrate, a plate-like diaphragm including a diaphragm portion, asupporting portion and a plurality of flexible displacement portions forcoupling the diaphragm portion and the supporting portion such that thediaphragm portion and the supporting portion become able to move inparallel to the substrate, an electret film disposed either on thesubstrate at its surface opposing the diaphragm or on the diaphragm atits surface opposing the substrate, and holding means for holding thesubstrate and holding the supporting portion of the diaphragm in such amanner that the diaphragm portion comes close to or away from thesubstrate in parallel. With this arrangement, the diaphragm is vibratedby vibrations applied to the vibration sensor and thereby vibrationsapplied to the vibration sensor can be detected.

According to a seventh embodiment, in the vibration sensor according tothe sixth embodiment, the plurality of flexible displacement portionsare radiated from a center of the diaphragm portion. With thisarrangement, the diaphragm is supported in a manner in which it can bevibrated satisfactorily.

According to an eighth embodiment, in the vibration sensor according tothe seventh embodiment, the diaphragm has opening portions formedbetween it and the plurality of flexible displacement portions. Withthis arrangement, the diaphragm can be satisfactorily vibrated byvibrations and thereby vibrations can be detected satisfactorily.

According to a ninth embodiment, in the vibration sensor according tothe eighth embodiment, the opening portions are disposed along acircumferential direction of the diaphragm portion. With thisarrangement, vibrations can be detected more satisfactorily.

According to a tenth embodiment, in the vibration sensor according tothe sixth embodiment, the holding means accommodates therein thesubstrate and the diaphragm in substantially closed state and holds thesubstrate and the supporting portion of the diaphragm. With thisarrangement, the inside of the sensor can be prevented from beingsmudged by dusts or the like. Moreover, the diaphragm can be preventedfrom being vibrated by a sound pressure based on sounds applied to thevibration sensor. Therefore, vibrations can be detected satisfactorily.

According to an eleventh embodiment, a vibration sensor comprises asubstrate, a plate-like diaphragm including a diaphragm portion, asupporting portion for supporting the diaphragm portion such that thediaphragm portion becomes able to move in parallel to the substrate andan opening portion formed between the diaphragm portion and thesupporting portion, the plate-like diaphragm being opposed to thesubstrate with a predetermined interval and being opposed in parallel tothe substrate, an electret film disposed either on the substrate at itssurface opposing the diaphragm or on the diaphragm at its surfaceopposed to the substrate and holding means for holding the substrate andholding the supporting portion of the diaphragm in such a manner thatthe diaphragm portion comes close to or away from the substrate inparallel. With this arrangement, the diaphragm is vibrated by vibrationsapplied to the vibration sensor and thereby vibrations applied to thevibration sensor can be detected.

According to a twelfth embodiment, in the vibration sensor according tothe eleventh embodiment, the opening portion is disposed along acircumferential direction of the diaphragm. With this arrangement,vibrations can be detected satisfactorily.

According to a thirteenth embodiment, in the vibration sensor accordingto the eleventh embodiment, the holding means is a housing foraccommodating therein the substrate and the diaphragm in substantiallyclosed state and holding the substrate and the supporting portion of thediaphragm. With this arrangement, the inside of the sensor can beprevented from being smudged by dusts or the like. Moreover, thediaphragm can be prevented from being vibrated by a sound pressure basedon sounds applied to the vibration sensor and thereby vibrations can bedetected satisfactorily.

According to a fourteenth embodiment, a navigation apparatus comprisesposition detecting means for detecting a current position of anavigation object, a vibration sensor comprising a plate-like diaphragmcomposed of a substrate, a diaphragm portion, a supporting portion, anda plurality of flexible displacement portions for coupling the diaphragmportion and the supporting portion such that the diaphragm portion andthe supporting portion become able to move in parallel to the substrate,the plate-like diaphragm being opposed to the substrate with apredetermined interval and being placed in parallel to the substrate, anelectret film being disposed either on the substrate at its surfaceopposing the diaphragm or on the diaphragm at its surface opposing thesubstrate, and holding means for holding the supporting portion of thediaphragm in such a manner that the diaphragm comes close to or awayfrom the substrate in parallel, reproducing means for reading andreproducing map data, display means for displaying thereon the map datasupplied thereto from the reproducing means, and control means forcontrolling an operation of the reproducing means based on an outputsignal from the vibration sensor and/or an output signal from theposition detecting means. With this arrangement, a vibrated state of theposition at which the navigation apparatus is installed can be detectedbased on the detected output of the vibrations sensor. Therefore, anoperation of the navigation apparatus can be controlled satisfactorily.

According to a fifteenth embodiment, a navigation apparatus comprisesposition detecting means for detecting a current position of anavigation object, a vibration sensor comprising a plate-like diaphragmcomposed of a substrate, a diaphragm portion, a supporting portion forsupporting the diaphragm portion such that the diaphragm portion becomesable to move in parallel to the substrate and an opposing portiondisposed between the diaphragm portion and the supporting portion, theplate-like diaphragm being opposed to the substrate with a predeterminedinterval and being placed in parallel to the substrate, an electret filmbeing disposed either on the substrate at its surface opposing thediaphragm or on the diaphragm at its surface opposing the substrate, andholding means for holding the substrate and holding the supportingportion of the diaphragm in such a manner that the diaphragm portioncomes close to or away from the substrate in parallel, reproducing meansfor reading and reproducing map data, display means for displayingthereon the map data supplied thereto from the reproducing mean, andcontrol means for controlling an operation of the reproducing meansbased on an output signal from the vibration sensor and/or an outputfrom the position detecting means. With this arrangement, the vibratedstate of the position at which the navigation apparatus is installed canbe detected based on the detected output of the vibration sensor.Therefore, an operation of the navigation apparatus can be controlledsatisfactorily.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrative of a principle of avibration sensor;

FIG. 2 is a sound pressure frequency characteristic graph of aclosed-type vibration sensor;

FIG. 3 is a frequency characteristic graph showing a sound pressureresponse of a diaphragm with a through-hole defined therein;

FIG. 4 is a frequency characteristic graph showing a vibration responseof a diaphragm with a through-hole defined therein;

FIG. 5 is a cross-sectional view illustrating a vibration sensoraccording to an embodiment of the present invention;

FIG. 6 is an exploded perspective view illustrating a main portion ofthe vibration sensor according to the embodiment in exploded form;

FIG. 7 is a plan view showing a diaphragm of the vibration sensoraccording to the embodiment;

FIG. 8 is a circuit diagram showing an equivalent circuit of thevibration sensor according to the embodiment;

FIG. 9 is a frequency characteristic graph of the vibration sensoraccording to the embodiment;

FIG. 10 is a block diagram showing a navigation apparatus to which thereis applied the vibration sensor according to the embodiment; and

FIG. 11 is a plan view showing a modified example of a vibration sensor.

BEST MODE FOR CARRYING OUT THE INVENTION

A vibration sensor according to the present invention will hereinafterbe described in detail with reference to the drawings.

FIG. 5 shows in cross-sectional form a vibration sensor according to theinvention. A case 1 is a cylindrically-shaped case made of a materialsuch as aluminum Al. One end of the case is closed to form a bottomportion 1b and the other end is opened. The case 1 houses thereinassembly elements shown in FIG. 6. A diaphragm ring 2 is disposedbetween a diaphragm, which will be described later on, and the bottomportion 1b of the case 1 are annularly-shaped by use of synthetic resinmaterial or the like. The diaphragm ring 2 has an opening portion 2a ofsubstantially the same size as an inner diameter of a supporting portionof the diaphragm which will be described later on.

A diaphragm 3 is composed of a diaphragm portion 3a, a supportingportion 3b, a plurality of flexible displacement portions 3c and aplurality of opening portions 3d as shown in FIG. 6. The diaphragm is adisk-shaped diaphragm made of a metal material such as stainless steel.By way of example, as shown in FIG. 7, the diaphragm 3 is formed of 40μm-thick stainless steel such that an outer diameter φ₁ of the wholediaphragm is about 9 mm, an inner diameter φ₂ of the supporting portion3b is about 7 mm, an outer diameter φ₃ of the diaphragm 3a is about 6 mmand a width of the flexible displacement portion 3c is about 2 mm. Theplurality of flexible displacement portions 3c are narrow and radiatedfrom the center of the diaphragm 3a at equal intervals. According tothis embodiment, although there are six flexible displacement portions3c between the diaphragm portion 3a and the supporting portion 3b asshown in FIG. 6, the number of the flexible displacement portions 3c isnot limited thereto and at least more than three flexible displacementportions 3c may be sufficient for enabling the diaphragm portion 3a tobe displaced or moved in parallel to a back plate which will bedescribed later on. A plurality of opening portions 3d are disposedbetween the flexible displacement portions 3c at equal intervals in thediaphragm portion 3a and the supporting portion 3b such that long axesthereof are extended along the circumferential direction of thediaphragm portion 3a. These opening portions 3d are disposed in order toprevent the diaphragm 3 from being vibrated by a sound pressure based onsounds applied to the vibration sensor from the outside. If a pluralityof opening portions are provided along the circumferential direction ofthe diaphragm portion 3a as shown in FIG. 7, then the area in which thediaphragm portion 3a and the electret film oppose to each other can beprevented from being reduced unlike the aforesaid vibration sensorhaving the diaphragm with the through-hole defined at its centralportion. As a consequence, the vibration sensor can be prevented fromdetecting sounds from the outside and therefore the detectionsensitivity of the vibration sensor can be prevented from being lowered.The diaphragm 3 has an epoxy resin insulating layer having a thicknessof about 1 μm on its surface opposing the back plate which will bedescribed later. According to this embodiment, since the diaphragm 3detects vibrations generated between it and the monitored object, thediaphragm is made of a material having a relatively large mass, e.g.,material of metal such as stainless steel.

A spacer 4 is a ring-shaped spacer made of a film-shaped synthetic resinhaving a thickness of about 40 μm. The spacer 4 includes an openingportion 4a as shown in FIG. 6. The opening portion 4a is as large asabout the inner diameter of the supporting portion 3b of the diaphragmsimilarly to the opening portion 2a of the diaphragm ring 2. A backplate 5 is opposed to the diaphragm 3 through the spacer 4. The backplate 5 is formed of a circular metal plate having a diameter largerthan that of the diaphragm portion 3a of the diaphragm 3. The back plate5 has an electret film 5b on its surface opposing the diaphragm 3 andalso has a plurality of holes 5a.

A cylindrical frame 6 with an opening portion 6a formed at its centralportion made of a synthetic resin material is attached to the peripheralportion of the back plate 5. The frame 6 has formed on its one end arecess portion 6b large enough to surround the back plate 5. The bottomsurface of the recess portion 6b is communicated with the openingportion 6a. The opening portion 6a accommodates therein a field-effecttransistor (FET) 8 wherein one terminal 8b is connected, through anopening 7a in the circuit substrate 7, to a the circuit substrate 7 as aterminal board by a solder 9. Another terminal 8a of the FET 8, i.e.,gate is connected to the diaphragm 3 of the back plate 5 on its surfaceopposite to the opposing surface of the diaphragm 3. The FET 8 is usedto detect how much a capacity of electric charges accumulated betweenthe diaphragm 3 and the back plate 5 is changed when the diaphragm 3 isvibrated or displaced relative to the back plate 5. The other end 1a ofthe case 1 is caulked to press the circuit substrate 7 down and the casea is almost closed under the condition that the case 1 accommodates therespective assembly elements. Since the case 1 is almost closed, theinside of the case 1 can be prevented from being smudged by dust or thelike. Moreover, the case 1 is placed in the closed state enough forpreventing the diaphragm 3 from being vibrated by a sound pressure basedon sounds applied to the vibration sensor.

The supporting portion 3b of the diaphragm 3 is held by the frame 6through the diaphragm ring 2 and the spacer 4. As a consequence, thediaphragm portion 3 is supported by a plurality of flexible displacementportions 3c in parallel to the back plate 5 such that it can bedisplaced or vibrated. If a plurality of opening portions 3d are formedon the diaphragm 3, then it is possible to prevent the diaphragm 3 frombeing vibrated by a sound pressure based on sounds applied to thevibration sensor. Since the diaphragm 3 has almost no electric chargesaccumulated at its portion adjacent to the supporting portion 3b andsuch adjacent portion is hardly vibrated as compared with the diaphragmportion 3a, a vibration detection sensitivity can be prevented frombeing degraded by the opening portion 3d formed at the portion whereinthe diaphragm portion 3a and the supporting portion 3b are joined.

In the above vibration sensor, the diaphragm 3 and the back plate 5 havetherebetween accumulated electric charges of capacity corresponding to asize of a clearance provided between the diaphragm 3 and the back plate5. When the diaphragm 3 is vibrated relative to the back plate, theclearance between the diaphragm 3 and the back plate 5 is changed,thereby changing the capacity of electric charges accumulated betweenthe diaphragm 3 and the back plate 5. When the FET 8 detects the changeof capacity, the vibration sensor generates an electrical signalcorresponding to the vibrations of the diaphragm. Thus, it is possibleto obtain the output signal serving as a detected result of vibration.

If the diaphragm of the vibration sensor is fixed as in the vibrationsensor shown in FIG. 1, when the vibration sensor and the monitoredobject with the vibration sensor attached thereto, e.g., a vehicle, arevibrated, the vibration sensor is vibrated with vibrations. In otherwords, because the back plate 5 of the vibration sensor is vibratedrelative to the diaphragm 3, the capacity of electric chargesaccumulated between the diaphragm 3 and the back plate 5 is changed. Thevibration sensor can obtain the output signal corresponding tovibrations by detecting the change of capacity with the FET 8.Conversely, since considering the state that the vibration sensor isfixed together with the detected object, if a vibration is applied tothe vibration sensor, the diaphragm 3 is vibrated relative to the backplate 5, thereby changing the capacity of electric charges accumulatedbetween the diaphragm 3 and the back plate 5. The vibration sensor canoutput the output signal corresponding to the vibration by detecting thechange of the capacity with the FET B. In that case, even when soundsare applied to the vibration sensor from the outside, a plurality ofopening portions 3d disposed on the diaphragm 3 can prevent thediaphragm 3 from being vibrated. Therefore, the vibration sensor cansatisfactorily detect only vibrations applied to the vibration sensor.

FIG. 8 shows an equivalent circuit of the vibration sensor according tothis embodiment. The diaphragm portion 3a of the diaphragm 3 shown by m,c₁, r generates an equivalent impedance for a sound pressure P. Sincethe diaphragm 3 has the opening portions 3d defined therein, animpedance z' based on the opening portions 3d is connected to theimpedance of the diaphragm portion 3a in parallel. I assumes a speed ofthe vibration unit 3 of the diaphragm 3. Then, a displacement x of thediaphragm portion 3a of the diaphragm 3 is expressed by the followingequation: ##EQU1##

As described before, when the distance between the diaphragm 3 and theback plate is changed, the electrostatic capacity of the capacitorcomposed of the diaphragm 3 and the back plate 5 is changed. As aconsequence, the displacement of the diaphragm 3 is detected as thechange of the electrostatic capacity. Therefore, the output signal fromthe vibration sensor is proportional to the displacement x of thediaphragm portion of the diaphragm 3.

FIG. 9 shows a frequency characteristic of the vibration sensoraccording to this invention. A characteristic denoted by f₁ shows afrequency characteristic of the vibration sensor according to thisinvention. Since the vibration sensor according to this invention is ofthe type that the diaphragm portion 3a and the supporting portion 3c ofthe diaphragm 3 are coupled by a plurality of flexible displacementportions 3e, a compliance of the diaphragm 3 can be raised. When thecompliance of the diaphragm 3 is raised, this is equivalent to the factthat the edge supporting a diaphragm of an ordinary speaker is softened.In the diaphragm 3 according to this embodiment, the diaphragm portion3a and the supporting portion 3b are coupled by a plurality of flexibledisplacement portions with small widths so that the diaphragm portion 3acan be easily vibrated. Therefore, when the compliance of the diaphragmis raised, this can achieve substantially similar effects that thespeaker edge is softened. As a result, according to the vibration sensoraccording to the present invention, the output level thereof can beraised higher than the frequency characteristic f₂ of the conventionalvibration sensor. The characteristic of high band portion can beimproved as compared with a frequency characteristic f₃ of the vibrationsensor having substantially the same output level of this embodiment. Aweight of the diaphragm of the vibration sensor according to thisembodiment can be reduced with the result that the peak portion of thehigh band portion of the frequency can be shifted to a higher frequency.Also, the level of the peak portion can be lowered.

A navigation apparatus using the vibration sensor according to theembodiment of the present invention will be described with reference toFIG. 10. The navigation apparatus shown in FIG. 10 is for use as anautomobile navigation apparatus.

In FIG. 10, the navigation apparatus comprises a vibration sensor 10, aspeed detecting circuit 11, a controller 12, an antenna 13, a currentposition detecting circuit 14, a map data making circuit 15, areproducing apparatus 16, a display apparatus 17 and a console 18.

The vibration sensor 10 is the vibration sensor shown in FIGS. 5 and 6.The vibration sensor 10 detects an acceleration of the vehicle. Anoutput signal indicative of an acceleration from the vibration sensor 10is supplied to the speed detecting circuit 11. The speed detectingcircuit 11 supplies an output signal indicative of a current speed ofvehicle being used as a mobile object by integrating the output signalsupplied thereto from the vibration sensor 10.

On the other hand, the controller 12 is supplied with an output signalfrom the current position detecting circuit 14. The current positiondetecting circuit 14 measures the current position of the vehicle basedon a signal received at the antenna 13 from a position measuringsatellite, and supplies data concerning the current position to thecontroller 12. When the position of the vehicle is measured by thesatellite, a GPS (Global Positioning System) is used. The controller 12controls operation of the map data making circuit 15 based on datasupplied thereto from the current position detecting circuit 15 and/orthe speed detecting circuit 11 to thereby make map data to be displayedon the display apparatus 17.

The map data making circuit 15 reproduces data corresponding to dataconcerning the vehicle current position obtained by the calculation fromthe current position detecting circuit 14 from a CD-ROM by supplying acontrol signal to the reproducing apparatus which reproduces the CD-ROMserving as a recording medium with map data recorded therein andgenerates map data based on data thus read out. The reproducingapparatus 16 includes a optical head for reading data recorded on theCD-ROM, a rotation drive mechanism for rotating the disk and a signalprocessing circuit for effecting a signal processing such aserror-correction on the data read out from the disk.

The display apparatus 17 is composed of liquid-crystal display device orthe like. The display apparatus displays map information based on themap data made by the map data making circuit 15. The console 18 iscomposed of a plurality of operation keys. When the keys on the consoleare operated, various operation mode setting or switching data, such asdata for accessing map displayed on the display apparatus 17 or data forswitching display modes are supplied to the controller 12.

In the navigation apparatus, the current position detecting circuit 14detects data concerning the automobile's current position by calculationbased on the signal received at the antenna 13 from the satellite. Thecurrent position data thus obtained is supplied to the controller 12.under control of the controller 12, the reproducing apparatus reproducesdata from the CD-ROM corresponding to the current position data suppliedfrom the current position detecting circuit 14 and supplies read outdata to the map data making circuit 15, which is converted to map dataand displayed on the display apparatus 17 through the controller 12.When it is detected on the basis of the output from the velocity sensor10 that the automobile is not stopped or it is detected on the basis ofthe output from the speed detecting circuit 11 that the automobile isrunning at a speed higher than a predetermined speed, the controller 12is disabled to receive input data (all operation input data or onlyrelatively complicated operation input data) from the console 18 or suchdata are made invalid. Accordingly, under the condition that theautomobile is running, the driver cannot switch the operation mode ofthe navigation apparatus or switch map information displayed on thedisplay apparatus 17 by the console 18. Therefore, it is possible toprevent an accident from being caused when the driver switches theoperation mode of the navigation apparatus or map information duringdriving. Further, if the vibration sensor shown in FIGS. 5 and 6 is usedas the vibration sensor 10, then it is possible to prevent thenavigation apparatus from malfunctioning under control of the controller12 as a result of the vibration sensor detecting sounds generated in theautomobile compartment, such as conversations of passengers or soundsgenerated in the outside of the vehicle.

While the diaphragm portion 3a and the supporting portion 3b of thediaphragm 3 are coupled by a plurality of flexible displacement portions3c each radiated from the center of the diaphragm portion 3a instraight-line as described above, the above shape may be modified asshown in FIG. 11. FIG. 11 shows a modified example of the diaphragm 3. Adiaphragm 20 shown in FIG. 11 includes a disk-like diaphragm portion 21,a supporting portion 22, a plurality of flexible displacement portions23 and a plurality of opening portions. These elements are integrallyformed as an integrated disk by use of a material such as a metalmaterial or the like. A plurality of flexible displacement portions 23are extended at an equal interval along the tangential line of thedisk-like diaphragm portion 21. A plurality of opening portions 24 areformed along the circumferential direction of the diaphragm portion 21among the diaphragm portion 21, the supporting portion 22 and aplurality of flexible displacement portions 23 at equal intervals.

In the case of the diaphragm 20 shown in FIG. 11, similarly to thediaphragm 3 shown in FIG. 7, since the opening portion 24 is formedwithout reducing the area of the portion opposing the electret film 5b,i.e., the area of the diaphragm portion 21, a vibration detectionsensitivity can be prevented from being degraded. Further, since thediaphragm 20 can be prevented from being vibrated with sounds applied tothe vibration sensor, the vibration sensor can satisfactorily detectvibrations.

As set forth above, since the supporting portion of the diaphragm of thevibration sensor and the diaphragm portion are coupled by a plurality offlexible displacement portions disposed such that the diaphragm portionbecomes able to move in parallel to the back plate, the vibration sensorcan satisfactorily detect vibrations without lowering a sensitivity atwhich vibrations are detected and without vibrating the diaphragm bysound pressure applied to the vibration sensor.

The vibration sensor according to the present invention can be modifiedvariously without departing from the gist of the present invention.

While the electret film, for example, is disposed on the back plate inthe vibration sensor, the present invention is not limited thereto andthe electret film may be disposed on the diaphragm at its surfaceopposing the back plate.

Further, while the vibration sensor is of the type using the FET todetect how much the capacity of electric charges are changed with thechange of clearance provided between the diaphragm and the back plate asdescribed above, the present invention is not limited thereto and thepresent invention can be applied to a vibration sensor of the type usinga capacitor formed between a diaphragm and a substrate serving a backelectrode so as to apply a bias voltage to the portion between thediaphragm and the back electrode.

Incidentally, it is needless to say that the vibration sensor accordingto the present invention is not limited to the navigation apparatus andcan be applied to a wide variety of apparatus.

We claim:
 1. A vibration sensor comprising:a substrate; a disk-shapeddiaphragm from which are integrally formed a diaphragm portion, asupporting portion and a plurality of flexible displacement portions,wherein said displacement portions extend radially at equal intervalsaround a circumference of said diaphragm portion between said diaphragmportion and said supporting portion such that said diaphragm portion isable to move relative to said substrate, said diaphragm being spacedfrom said substrate by a predetermined interval to form a capacitor; andholding means for holding said substrate and holding said supportingportion of said diaphragm spaced at said predetermined interval, whereina vibration creates a change of electrostatic capacitance between saiddiaphragm and said substrate; wherein said holding means is a housingfor accommodating therein said substrate and said diaphragm and holdingsaid substrate and said supporting portion of said diaphragm; andwherein said holding means comprises a diaphragm ring, a spacer elementand a cylindrical frame.
 2. A vibration sensor according to claim 1,wherein said diaphragm includes, between said plurality of flexibledisplacement portions, means for canceling a sound pressure applied tosaid vibration sensor.
 3. A vibration sensor according to claim 2,wherein said means for canceling a sound pressure comprises a pluralityof opening portions disposed at equal intervals around saidcircumference of said diaphragm portion and disposed respectivelybetween adjacent flexible displacement portions.
 4. A vibration sensoraccording to claim 1, wherein said disk-shaped diaphragm is made of ametal material.
 5. A vibration sensor according to claim 4, wherein saidmetal material is 40 μm-thick stainless steel.
 6. A vibration sensoraccording to claim 1, wherein said spacer element is made of afilm-shaped synthetic resin having a thickness of about 40 μm.
 7. Avibration sensor according to claim 1, wherein an FET within said sensoris used to detect the change of electrostatic capacity formed betweensaid diaphragm and said substrate.
 8. A vibration sensor according toclaim 1, wherein said flexible displacement portions are disposed at anangle to a radial direction of said diaphragm portion.
 9. A vibrationsensor according to claim 1, wherein a distance between said diaphragmportion and said supporting portion is less than a length of saidflexible displacement portions.